They're tricky.
I've discovered this year that a "women's medium" from Primal is equivalent to a "women's large" from Pactimo. I was expecting this a bit, because people on the Internet were saying that Pactimo runs small (possibly due to using international sizing, and Americans are unusually large).
Not that I'm especially easy to fit--I don't look much what I'd expect a serious cyclist to look like. But if you ever needed a cross-brand guideline...there ya go.